FIG. 8 represent a unit for the moulding of plastic material, of conventional type and generally indicated with 300, which comprises a press or moulding machine 301.
The moulding machine 301 in turn is provided with a fixed part or plate 302, and with a mobile part or plate 303, wherein the mobile plate 303 is suitable for moving in horizontal sense, as represented with dash and dot line and pointed out by a double arrow f10, with respect to the fixed plate 302, to the purpose of closing with a determined closing force F1, during the moulding step of a piece P, a mould 304, which is interposed between the two fixed and mobile plates 302, 303.
The mould 304 is generally constituted by two or more parts, indicated with 304a, suitable for being separated the one from the other, when the mobile plate 303 is moved away from the fixed plate 302, so as to allow the extraction of the piece P, once moulded, from the mould 304.
A distribution assembly 306, also of known type, is associated with the fixed plate 302 of the moulding machine 301 and has the function of receiving from a feeding unit, not represented, through an opening 302a formed in the fixed plate 302, a plastic material MP, in the fluid state, as schematized with a corresponding arrow MP, so as to convey and distribute it to the various cavities defined inside the mould 304, to the purpose of forming the moulded piece P.
Therefore, in the moulding unit 300, the distribution assembly 306, associated with the fixed plate 302 of the moulding machine 301, is subject to the closing force F1 determined, during the moulding step of the piece P, by the closure of the mobile plate 303 against the fixed plate 302, with the interposition of the mould 304.
In the detail, this distribution assembly 306 comprises a bottom plate 307; a nozzle-bearing plate 308 bearing a plurality of injection nozzles 311, called also in English “nozzles”, arranged in correspondence of those areas of the mould 304 in which the plastic material MP has to be injected; and an intermediate plate 309, also called in English “hot plate” or “manifold”, which is interposed between the bottom plate 307 and the nozzle-bearing plate 308 and defines internally a network of flow channels 312.
The intermediate plate 309 is arranged between the external bottom plate 307 and the nozzlebearing plate 308, which are usually cooled and therefore maintained at a temperature lowest than that of the distribution plate 309, in such as way to be free to expand longitudinally relatively to them, in order not to activate any harmful stresses and strains in consequence of the heating during the use of the distribution assembly 306.
The distribution plate 309 is arranged between the external bottom plate 307 and the nozzle-bearing plate 308, which are usually cooled and therefore maintained at a temperature lowest than that of the distribution plate 309, in such as way to be free to expand longitudinally relatively to them, in order not to activate any harmful stresses and strains in consequence of the heating during the use of the distribution assembly 306.
In detail, the plain surface of the intermediate distribution plate 309 is solely coupled by a contact way, i.e. by a slidable contact exempted from clearance, with the plain surfaces of the ends 311a of the injection nozzles 311, to which it distributes the fluid plastic material MP.
Therefore, the intermediate plate 309 can freely slide with respect to such ends 311a, when it is subject to expand longitudinally, or to become “hot”, because of the heating induced by the activation of heater elements, as electric resistances, arranged at its inside.
It is fundamental, in the solutions at the moment known and applied, that the distribution assembly 306, in order to work correctly, be integrated into the mould 304 in such a way to be subject to the closing force F1, exerted by the moulding machine 301 during the moulding of the piece P.
In fact, only with the contribution and the presence of the closing action exerted by this closing force F1, as it has been ascertained experimentally, it is possible to carry out a correct functionality of the distribution assembly 306, and in particular to obtain a perfect sealing against any outflow towards the outside, in particular in the area of the slidable contact and coupling with the ends of the injection nozzles 311, of the fluid plastic material MP which flows through the network of channels 312 towards the injection nozzles 311, despite the mutual sliding caused by the different thermal expansions occurring between the intermediate plate 309, from one side, and the nozzle-bearing plate 308 with the respective injection nozzles 311, from the other side.
The schemes of FIGS. 9 and 9a refer instead to a more general plant 400, also of conventional type, in which a unit or moulding machine 401, for the moulding of plastic material, is associated with a blowing unit of known type, not represented in the drawings.
The moulding unit 401 has the function of moulding a preformed piece of plastic material, usually of hollow shape, which is subsequently transferred to the blowing unit, where a powerful blow of compressed air is blown inside it, still warm and deformable, whereby the preformed piece expands and acquires a shape corresponding to that final.
In this plant of conventional type the moulding unit 401 comprises a press 402 which is provided with a mobile part or plate 402a, in turn suitable for moving in vertical sense, as represented by a double arrow f11, relatively to a fixed part or plate 402b, to the purpose of closing, from two opposite sides and by exerting a determined closing force F2, a mould 404, schematically represented with dash and dot line, which is interposed between the two mobile and fixed plates, 402a and 402b, and is composed of two or more separable parts.
A distribution and injection assembly or block 406 is associated with the fixed structure of the moulding machine 401 and has the function of receiving the fluid plastic material MP, supplied by a supply group 424, for distributing and injecting it into the mould 404, so as to form the moulded preformed piece.
This distribution assembly 406 is arranged along a side 404c, of the mould 404, which is not associated with the fixed and mobile plates, respectively 402b and 402a, of the press 402, whereby the distribution assembly 406 is not subject to the closing force F2 which is exerted by the latter on the mould 404 in order to close it during the injection and moulding step.
In detail the distribution and injection assembly 406 comprises a bottom plate 407 and a nozzle-bearing plate 408, which bears a plurality of injection nozzles 411, that are positioned in correspondence of those areas of the mould 404 in which the plastic material MP has to be injected.
The bottom plate 407 and the nozzle-bearing plate 408 are arranged one against the other and define along a common contact surface 409 a network of hot channels 412 suitable for conveying the fluid plastic material MP towards the injection nozzles 411.
Both the bottom plate 407 and the nozzle-bearing plate 408 are associated with electric resistances, housed at their inside, which have the function of bringing and maintaining them at a high temperature, while the plastic material MP flows along the channels 412, whereby these plates 407 and 408 are subject to a considerable heating and to relevant variations of temperature during the use.
In turn the nozzles 411 are each rigidly fixed at a respective end to the nozzle-bearing plate 408, whereby they are subject to follow the thermal expansions of the nozzle-bearing plate 408, when it heats in the use.
It follows that this known solution, shown with reference to the FIGS. 9 and 9a, implies the not negligible drawback that, because of the thermal expansions of the nozzle-bearing plate 408, the nozzles 411 often are subject to shift from their nominal positions.
In this way the nozzles 411 may force against other parts of the moulding system, so as to bend and thereby assume a not correct and imprecise arrangement for injecting the plastic material into the mould, or even only to activate dangerous stresses.